The present invention relates to a fuel assembly adapted for use in nuclear reactors.
In boiling water reactors, skewing of power distribution is caused due to uneven void distribution in the axial direction of the reactor.
More specifically, while the bottom of the reactor core at which the inlet for the coolant is located is kept in subcooling of about 10 kcal/kg, boiling and saturated boiling take place at the medium and upper portions of the core. Thus, the void fraction is increased as it goes upward, and reaches 70% or so at the top of the reactor core where the outlet for the coolant is provided. Consequently, the neutron thermalization is more remarkable at the bottom portion than at the top portion of the reactor core, to cause a skewed pattern of power distribution, so as to locate the point of the power peak at a lower position of the reactor core, typically at 1/4 of the whole core height as measured from the bottom of the latter. In addition, the peak itself is steeper and high, as compared with that observed in other types of reactors.
To cope with this problem, there have been proposed a variety of solutions. According to one of these conventional measures, specific control rods generally referred to as "shallow rods" are inserted to the core at a portion of the latter 1/4 of the entire core height from the bottom, where the power peak is formed.
In another solution, fuel rods provided with gadolinea (Gd.sub.2 O.sub.3) at their portions corresponding to the position of the power peak are used.
In the power distribution control by means of the shallow rods, the power peaking is formed around the ends of these rods, so that the fuel rods have to experience an abrupt change of power when the control rods are withdrawn. The shallow rods are often used in combination with so-called deep rods adapted to control the radial power distribution and reactivity of the reactor core. In such a case it is necessary to calculate various patterns of positions of the control rods, for flattening the axial and radial power distribution of the reactor, which is extremely difficult and troublesome.
In case of the gadolinea-containing fuel rods, the power distribution is largely affected by the position of the gadolinea. Thus, a slight design error of the fuel rod often renders the control of the power distribution difficult, and the power distribution is largely changed in accordance with the change of exposure.
For the reasons as stated above, these conventional measures have been ineffective for practical control of the nuclear reactor of the kind described.